Obvious

In mathematics the word obvious is a commonly-used subjective term meaning something like, "I'm pretty sure this is right and that nothing really interesting would be learned if you took the trouble of proving that it was."

It therefore corresponds to the apparent "geek" use of trivial, which I think has a narrower and less subjective meaning within mathematics itself: see that node for more, but in brief, using factorial as an example, 0! = 1 is the degenerate case, 1! = 1 is the trivial case, and everything from 2! upward is non-trivial but obvious.

Obviousness does not lie in seeing the answer immediately, only the fact that the answer is eventually attainable without any great mental effort. It is legendary that a mathematician will assert in a hand-waving way that some point is obvious, and on being challenged, will work away at it privately for ages before confirming the obviousness. Or they will write it in the book and it is the poor student who has to work for ages to get it. The great G.H. Hardy, on being challenged, left the classroom and went for a twenty-minute* walk to think about it. On returning he said, "Yes, I was right, it is obvious", and carried on.

www.langston.com/Fun_People/1999/1999ADT.html
math.cofc.edu/jokes.html mentions 'obvious' in one

* insert any number here depending on iterations of the joke before it reaches you